LEEUWENHOEK 203 



inch in diameter. Dr. James Jurin, a London physician, 

 improved upon this. A fine silver wire was wound in a 

 close spire about a slender cylinder, such as a pin ; the 

 length of the spire divided by the number of the 

 turns gave the diameter of the wire. Short lengths 

 of this wire, strewn about the field of view, served as 

 measures of length.^ Benjamin Martin adopted an 

 expedient which is still much used, that of a glass disk, 

 ruled with fine lines and fitted in the focus of the eye- 

 glass.^ 



The Tadpole^ 



The circulation occupies a large part of the letter, but 

 Leeuwenhoek mentions some interesting details con- 

 cerning other parts of the frog's history. He notes one 

 use of the jelly which coats the eggs, viz. that it becomes 

 loaded with minute air-bubbles, which, by rendering the 

 eggs buoyant, expose them to the heat of the sun. The 

 mouth of the tadpole with its many rows of teeth, the 

 suckers and the external gills are described. Like 

 Swammerdam, whose account was not yet printed,* 

 Leeuwenhoek remarks that the young tadpole is entirely 

 composed of cells (" globules "). 



Blood and the Circulation^ 



Malpighi {supra, p. 161) had extended the knowledge 

 of the vascular system by showing that innumerable 

 capillary vessels connect the arteries and the veins in 

 the lung, the mesentery and the bladder of the frog. 



^ Phil. Trans., ISo. 355 (1118) and Dissert. phyaico-mcUh. 8vo. Lond. 1732, 

 pp. 45-6. 



2 New and compendious system of Optics. 8vo. Lond. 1732, pp. 45-6. 



SEpist. 65, Arc. Nat. (1688). Vol. II, pp. 163-172. 



* Supra, p. 197. 



^Phil. Trans., 1674 ; Epist. 65-8, Arc. Nat. (1688-91). Vol. II, pp. 153-217. 



